MIAMI — A hazardous new synthetic drug originating in China is being blamed for 18 recent deaths in a single South Florida county, as police grapple with an inexpensive narcotic that causes exaggerated strength and dangerous paranoid hallucinations.

On Thursday, the Fort Lauderdale police killed a man, reportedly high on the man-made street drug, alpha-PVP, known more commonly as flakka, who had held a woman hostage with a knife to her throat.

The shooting of Javoris Washington, 29, was the latest in a series of volatile episodes that the police in South Florida have faced with highly aggressive drug users. Law enforcement agencies have had difficulty tamping down a surge in synthetic drugs, which were banned after becoming popular in clubs five years ago only to re-emerge deadlier than ever under new formulations. As soon as legislation catches up with the latest craze, manufacturers design a new drug to take its place, federal and local law enforcement agencies say.

In Broward County, which includes Fort Lauderdale and is considered ground zero for the new drug, there have been 18 flakka-related fatalities since September, the chief medical examiner there said.

“I have never seen such a rash of cases, all associated with the same substance,” said James N. Hall, an epidemiologist at Nova Southeastern University who has studied the Florida drug market for decades. “It’s probably the worst I have seen since the peak of crack cocaine. Rather than a drug, it’s really a poison.”

Flakka, which got its name from a Spanish colloquial term for a pretty, enticing woman, is a synthetic cathinone that mimics the khat plant grown in Africa. It is made from alpha-pyrrolidinovalerophenone, what Mr. Hall describes as “second-generation bath salts,” a reference to previous formulations of the amphetaminelike stimulant.

Also known as gravel, flakka made a sudden and explosive entrance into South Florida’s illicit drug market about six months ago, particularly in poor neighborhoods, where drug users including homeless people were lured by the low price, $5 a dose.

Police departments around the state, and especially those near Fort Lauderdale, have been called to a growing number of situations involving people high on the drug who were convinced that packs of dogs or people were chasing them.

In February, a 50-year-old homeless man tried to kick in the glass door at the Fort Lauderdale Police Department because he believed people were chasing him. In Melbourne this month, a 17-year-old girl ran down the street naked and covered in blood, screaming that she was Satan.

In Broward County, a man ran down a street wearing only sneakers, saying a pack of German shepherds was hunting him. Another person became impaled on a fence.

“Police departments are always calling us for backup, because they try not to apprehend somebody on synthetic drugs by themselves,” said Mia Ro, a spokeswoman for the Drug Enforcement Administration’s Miami division.

At first, the products known as bath salts were available in gas stations. When specific chemical substances were banned in Europe and the United States, chemists tweaked the formula, and flakka emerged.

Five major synthetic cathinones were banned federally and by most states in 2010. Flakka is illegal in the United States, and law enforcement authorities are working with officials in China for it to be outlawed there as well.

“Our supposition is that the original concept was to design it so it would be technically not illegal,” Mr. Hall said. “It appears they are now looking to also design the molecule to be even more potent and more addictive. Addiction is good for sales.”

But the law has not stopped its flow, Mr. Hall said.

Broward County’s chief medical examiner, Dr. Craig Mallak, said the drug manufacturers added a ketone, an oxygen atom that affects more receptors in the brain.

The drug works by blocking the reuptake function of transmitting neurons, allowing a storm of dopamine and serotonin to flood the brain, Dr. Mallak said.

Flakka comes in the form of crystals of different colors that dissolve in the mouth, and the drug is also smoked and can be used for “vaping” in e-cigarette-like devices. The body temperature of users who take too much can rise above 105 degrees, resulting in excited delirium. Users can feel so hot that they may strip off their clothes. Some have suffered kidney failure and cognitive impairment.

“They do really wild things,” Dr. Mallak said. “A lot of them get hyperthermia and die of heat stroke. A few attack police officers, end up getting shot. They tear their clothes off and go crazy.”

Many of the drug’s users remain high for three days on a $5 dose the size of one-tenth of a packet of sweetener.

The Broward Sheriff’s Office said the county lab first detected the drug in January 2014. By the end of last year, the lab had encountered about 190 cases. From January to mid-April this year, the lab had analyzed more than 400 cases.

“The problem we have as law enforcement is that it came on the scene so fast,” said Detective William Schwartz, a narcotics investigator at the Broward Sheriff’s Office. “This isn’t a drug that’s proliferating in the clubs. We are seeing it destroy low-income neighborhoods.”

Detective Schwartz said a $1,500 kilogram is delivered to dealers by major international delivery services, making it “readily available to anyone who knows how to use a computer,” he said.

The dose is so tiny that the initial investment can yield 10,000 doses, sold at $3 to $5 each. The small dosage size also makes it easy to consume too much, with fatal results.

“It looks just like meth, heroin or cocaine, depending on the state it’s in on the street,” Detective Schwartz said.

But because the normal dose of cocaine or meth is so much higher than flakka’s dose, one-tenth of a gram, even a little too much puts the user in a state of excited delirium, Detective Schwartz said, “and there’s no way back.”

A version of this article appears in print on , on Page A12 of the New York edition with the headline: Police in Florida Grapple With a Cheap and Dangerous New Drug . Order Reprints | Today’s Paper | Subscribe